I knew the basic backstory of the San Patricios, but when I got inspired to write a short story featuring a character who was a member of the Battalion, I asked friends for recs on a good history of the unit. The Rogue's March was suggested by several folks. It's a great overview of the clashes between WASP Army officers in the 1840s and the German and Irish immigrants under their command. The progress of the Battalion from its formation through its demise is presented well.